Methods and system for manufacturing a redox flow battery system by roll-to-roll processing

ABSTRACT

Methods and systems are provided for manufacturing a membrane separator for a redox flow battery. In one example, the membrane separator is fabricate by a calendering process. The membrane separator may be configured with a polymer network to provide selectivity for ion transport across the membrane separator. The membrane separator may be further adapted with an integrated spacer in contact with a negative electrolyte.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 16/536,259, titled “METHODS AND SYSTEM FORMANUFACTURING A REDOX FLOW BATTERY SYSTEM BY ROLL-TO-ROLL PROCESSING”,and filed on Aug. 8, 2019. U.S. patent application Ser. No. 16/536,259claims priority to U.S. Provisional Application No. 62/717,633, titled“METHODS AND SYSTEM FOR MANUFACTURING A REDOX FLOW BATTERY SYSTEM BYROLL-TO-ROLL PROCESSING”, and filed on Aug. 10, 2018. The entirecontents of the above-listed applications are hereby incorporated byreference for all purposes.

FIELD

The present description relates generally to methods and systems forredox flow battery systems.

BACKGROUND AND SUMMARY

Redox flow batteries are suitable for grid scale storage applicationsdue to their capability for scaling power and capacity independently, aswell as for charging and discharging over thousands of cycles withreduced performance losses in comparison to conventional batterytechnologies. A power module of a redox flow battery system includes aseparator disposed in a battery cell between a negative electrode and apositive electrode. The separator is configured to separate reactionsoccurring at the negative electrode from reactions occurring at thepositive electrode within an electrode compartment of a battery cellcontaining electrolyte. More specifically, the separator may act as abarrier to mixing of electrolyte in a positive electrode section of thecompartment with electrolyte in a negative electrode section of thecompartment.

The separator may be a permeable membrane that allows ions to beexchanged across the membrane from the positive side of the electrodecompartment to the negative side, or vice versa. Furthermore, theseparator may be adapted to allow passage of some ionic species whileselectively blocking exchange of other ions. A flow of ionic chargecarriers across the separator may provide charge balance betweenelectrolyte on the positive and negative sides of the battery cell andsuppress short circuiting of the electrodes through the electrolytes.

The battery separator may be formed from a polymer as a base materialcoated with an ionomer layer. The ionomer layer hinders flow of specificions across the separator, thus maintaining an efficiency of thebattery. The ionomer may also be a polymer that comprises primarilyelectrically neutral repeating units with a fraction of ionized unitsbonded to the polymer backbone. As a result of its structure, theionomer may be endowed with unique physical properties suitable forinteractions with electrolytes in the redox flow battery.

However, the inventors herein have recognized issues with the abovesystems. In one example, costs associated with use of the ionomer-coatedseparator in the redox flow battery system may be prohibitive from amarketing standpoint. Due to the repetitive implementation of batterycomponents in a battery stack, the ionomer-coated separator maycontribute to 25% or more of the total cost of the redox flow batterypower module. In order to increase a commercial viability of the redoxflow battery, a lower cost alternative to the ionomer-coated separatormay be desired.

In one example, the issues described above may be addressed by a methodfor manufacturing a redox flow battery, comprising combining andtreating chemical ingredients of a membrane separator, forming a sheetfrom the combined chemical ingredients, molding ribs onto a surface ofthe sheet, and infiltrating the sheet with a cross-linked polymernetwork.

In this way, the redox flow battery system may be manufactured at lowercost and over a shorter duration of time. The separator, formed from alow-cost microporous membrane and functionalized with a cross-linkedpolymer gel, may be fabricated via a roll-to-roll (R2R) calendaringprocess that allows the separator to be produced with high throughput ina cost-efficient manner. The separator may perform as well or betterthan a separator comprising an ionomer coating and, as a result of thecalendaring R2R method of production, may additionally be integratedwith a negative electrode spacer for controlling electrolyte flowchannels associated with the negative electrode of the redox flowbattery. Independent fabrication of the negative electrode spacer isthereby precluded and manufacturing throughput is further increased.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an example redox flow battery system thatincludes bipolar plates and a membrane separator.

FIG. 2 shows a flow diagram of a first example of a roll-to-roll processfor manufacturing a bipolar plate for the redox flow battery system.

FIG. 3 shows a flow diagram of a second example of a roll-to-rollprocess for manufacturing a bipolar plate for the redox flow batterysystem.

FIG. 4 shows a prophetic graph comparing production cost ofmanufacturing the bipolar plate by a molding method and by theroll-to-roll process.

FIG. 5 shows a first example of a method for fabricating the bipolarplate.

FIG. 6 shows a second example of a method for fabricating the bipolarplate.

FIG. 7 shows a schematic diagram of an example of a roll-to-roll system.

FIG. 8 shows examples of monomers that may be cross-linked to formpolymers in pores of the membrane separator.

FIG. 9 shows examples of ribs that may be molded into the membraneseparator to control a spacing between the separator and a negativeelectrode.

FIG. 10 shows a schematic diagram of an example of a calendering processfor manufacturing the membrane separator with an integrated spacer.

FIG. 11 shows an example of a method for fabricating the membraneseparator with molded ribs by the calendering process.

FIG. 12 shows an example of a power module of the redox flow battery.

FIG. 12 is shown approximately to scale.

DETAILED DESCRIPTION

The following description relates to systems and methods formanufacturing bipolar plates for a redox flow battery. The redox flowbattery is shown in FIG. 1 with an integrated multi-chamber tank havingseparate positive and negative electrolyte chambers. The electrolytechambers may be coupled to one or more battery cells, each cellcomprising a negative electrode and a positive electrode. The positiveand negative electrolytes may be separated within each of the one ormore battery cells by a bipolar plate. The positive and negativeelectrolytes may also be separated within each of the one or morebattery cells by a membrane separator that selectively allows transportof ions across the separator to maintain charge balance across thebattery cells. A series of steps for fabricating a composite bipolarplate by a roll-to-roll process is illustrated in a first flow diagramin FIG. 2 . An alternative roll-to-roll process for fabricating thebipolar plate is shown in a second flow diagram in FIG. 3 . Theroll-to-roll processes of FIGS. 2 and 3 may result in reduced costsassociated with raw materials and processing time. A prophetic graphtracing unit cost of manufacturing the bipolar plate versus quantity ofbipolar plates produced is shown in FIG. 4 . The graph includes a plotrepresenting fabrication of the bipolar plate by a batch method, such ascompression molding, overlaid with a plot depicting fabrication of thebipolar plate by the roll-to-roll process to provide an estimateddifference in expense. FIGS. 5 and 6 show examples of methods forroll-to-roll production of the bipolar plate, corresponding to theprocesses described in FIGS. 2 and 3 respectively, as conducted on asystem configured with rollers and a series of processing steps,commencing with an input roll of material and ending with an output rollof an assembled product. A schematic diagram of an example of aroll-to-roll system that may be used to manufacture the bipolar plate isshown in FIG. 7 .

The separator may be formed from a number of components, including across-linked polymer with functional groups that provide ion selectivelywhen the cross-linked polymer is integrated into pores of the basestructure of the separator. Examples of functional groups of thecross-linked polymer that may allow selective ion transport are depictedin FIG. 8 . In addition to separating positive and negative electrolytesand controlling flow of ions therebetween, the separator may be adaptedwith a geometry that affects a spacing between the separator and thenegative electrode as well as interaction of negative electrolyte withthe negative electrode. For example, the separator may be molded, asshown in FIG. 9 , with a plurality of ribs on a side of the separator incontact with negative electrolyte, directing flow along a surface of thenegative electrode and channeling bubbles of hydrogen gas, formed duringoperation of the redox flow battery system, away from the electrodesurface. Fabrication of the separator, with molded ribs, may be achievedby a roll-to-roll calendering process. An example of a calenderingsystem that may be used to produce the separator is shown in a schematicdiagram in FIG. 10 . An example of a method for fabricating theseparator with the integrated cross-linked polymer and molded ribs bythe roll-to-roll calendaring process is shown in FIG. 11 , which mayallow for cost-effective manufacturing of the separator with highthroughput and increased battery performance. The redox flow batterysystem may include a power module that contains the bipolar plate,negative spacer, positive and negative electrodes, membrane separatorand positive and negative electrolytes as repeating units. An example ofthe power module is depicted in FIG. 12 .

FIGS. 2, 3, 7, 10, and 12 show example configurations with relativepositioning of the various components. If shown directly contacting eachother, or directly coupled, then such elements may be referred to asdirectly contacting or directly coupled, respectively, at least in oneexample. Similarly, elements shown contiguous or adjacent to one anothermay be contiguous or adjacent to each other, respectively, at least inone example. As an example, components laying in face-sharing contactwith each other may be referred to as in face-sharing contact. Asanother example, elements positioned apart from each other with only aspace there-between and no other components may be referred to as such,in at least one example. As yet another example, elements shownabove/below one another, at opposite sides to one another, or to theleft/right of one another may be referred to as such, relative to oneanother. Further, as shown in the figures, a topmost element or point ofelement may be referred to as a “top” of the component and a bottommostelement or point of the element may be referred to as a “bottom” of thecomponent, in at least one example. As used herein, top/bottom,upper/lower, above/below, may be relative to a vertical axis of thefigures and used to describe positioning of elements of the figuresrelative to one another. As such, elements shown above other elementsare positioned vertically above the other elements, in one example. Asyet another example, shapes of the elements depicted within the figuresmay be referred to as having those shapes (e.g., such as being circular,straight, planar, curved, rounded, chamfered, angled, or the like).Further, elements shown intersecting one another may be referred to asintersecting elements or intersecting one another, in at least oneexample. Further still, an element shown within another element or shownoutside of another element may be referred as such, in one example.

Hybrid redox flow batteries are redox flow batteries that arecharacterized by the deposit of one or more electro-active materials asa solid layer on an electrode. Hybrid redox flow batteries may, forinstance, include a chemical that plates via an electrochemical reactionas a solid on a substrate throughout the battery charge process. Duringbattery discharge, the plated species may ionize via an electrochemicalreaction, becoming soluble in the electrolyte. In hybrid batterysystems, the charge capacity (e.g., a maximum amount of energy stored)of the redox battery may be limited by the amount of metal plated duringbattery charge and may accordingly depend on the efficiency of theplating system as well as the available volume and surface areaavailable for plating.

As shown in FIG. 1 , in a redox flow battery system 10, a negativeelectrode 26 may be referred to as a plating electrode and a positiveelectrode 28 may be referred to as a redox electrode. A negativeelectrolyte within a plating side (e.g., a negative electrodecompartment 20) of a battery cell 18 may be referred to as a platingelectrolyte, and a positive electrolyte on a redox side (e.g. a positiveelectrode compartment 22) of the battery cell 18 may be referred to as aredox electrolyte.

Anode refers to the electrode where electro-active material loseselectrons, and cathode refers to the electrode where electro-activematerial gains electrons. During battery charge, the positiveelectrolyte gains electrons at the negative electrode 26; therefore thenegative electrode 26 is the cathode of the electrochemical reaction.During discharge, the positive electrolyte loses electrons; thereforethe negative electrode 26 is the anode of the reaction. Alternatively,during discharge, the negative electrolyte and negative electrode may berespectively referred to as an anolyte and anode of the electrochemicalreaction, while the positive electrolyte and the positive electrode maybe respectively referred to as a catholyte and cathode of theelectrochemical reaction. In contrast, during charge, the negativeelectrolyte and negative electrode may be respectively referred to asthe catholyte and cathode of the electrochemical reaction, while thepositive electrolyte and the positive electrode may be respectivelyreferred to as the anolyte and anode of the electrochemical reaction.For simplicity, the terms positive and negative are used herein to referto the electrodes, electrolytes, and electrode compartments in redoxbattery flow systems.

One example of a hybrid redox flow battery is an all iron redox flowbattery (IFB), in which the electrolyte comprises iron ions in the formof iron salts (e.g., FeCl₂, FeCl₃, and the like), wherein the negativeelectrode comprises metal iron. For example, at the negative electrode26, ferrous ion, Fe²⁺, receives two electrons and plates as iron metalon to the negative electrode 26 during battery charge, and iron metal,Fe⁰, loses two electrons and re-dissolves as Fe²⁺ during batterydischarge. At the positive electrode, Fe²⁺ loses an electron to formferric ion, Fe³⁺, during charge, and during discharge, Fe³⁺ gains anelectron to form Fe²⁺. The electrochemical reaction is summarized inequations (1) and (2), wherein the forward reactions (left to right)indicate electrochemical reactions during battery charge, while thereverse reactions (right to left) indicate electrochemical reactionsduring battery discharge:

Fe²⁺+2e−↔Fe⁰−0.44 V (Negative Electrode)  (1)

Fe²⁺↔2Fe³⁺+2e−0.77 V (Positive Electrode)  (2)

As discussed above, the negative electrolyte used in the all iron redoxflow battery may provide a sufficient amount of Fe²⁺ so that, duringcharge, Fe²⁺ can accept two electrons from the negative electrode toform Fe⁰ and plate onto a substrate. During discharge, the plated Fe⁰may then lose two electrons, ionizing into Fe²⁺, and may be dissolvedback into the electrolyte. The equilibrium potential of the abovereaction is −0.44 V and thus, this reaction provides a negative terminalfor the desired system. On the positive side of the IFB, the electrolytemay provide Fe²⁺ during charge, which loses electron and oxidizes toFe³⁺. During discharge, Fe³⁺ provided by the electrolyte becomes Fe³⁺ byabsorbing an electron provided by the electrode. The equilibriumpotential of this reaction is +0.77 V, creating a positive terminal forthe desired system.

The IFB provides the ability to charge and recharge its electrolytes incontrast to other battery types utilizing non-regenerating electrolytes.Charge is achieved by applying a current across the electrodes viaterminals 40 and 42. The negative electrode 26 may be coupled viaterminal 40 to the negative side of a voltage source so that electronsmay be delivered to the negative electrolyte via the positive electrode(e.g., as Fe²⁺ is oxidized to Fe²⁺ in the positive electrolyte in thepositive electrode compartment 22). The electrons provided to thenegative electrode 26 (e.g., plating electrode) can reduce the Fe²⁺ inthe negative electrolyte to form Fe⁰ at the plating substrate, causingit to plate onto the negative electrode 26.

Discharge can be sustained while Fe⁰ remains available to the negativeelectrolyte for oxidation and while Fe²⁺ remains available in thepositive electrolyte for reduction. As an example, Fe²⁺ availability canbe maintained by increasing the concentration or the volume of thepositive electrolyte to the positive electrode compartment 22 side ofcell 18 to provide additional Fe²⁺ ions via an external source, such asan external positive electrolyte tank 52. More commonly, availability ofFe⁰ during discharge may be an issue in IFB systems, wherein the Fe⁰available for discharge may be proportional to the surface area andvolume of the negative electrode substrate as well as the platingefficiency. Charge capacity may be dependent on the availability of Fe²⁺in the negative electrode compartment 20. As an example, Fe³⁺availability can be maintained by providing additional Fe²⁺ ions via anexternal source, such as an external negative electrolyte chamber 50 toincrease the concentration or the volume of the negative electrolyte tothe negative electrode compartment 20 side of cell 18.

In an IFB, the positive electrolyte comprises ferrous ion, ferric ion,ferric complexes, or any combination thereof, while the negativeelectrolyte comprises ferrous ion or ferrous complexes, depending on thestate of charge of the IFB system. As previously mentioned, utilizationof iron ions in both the negative electrolyte and the positiveelectrolyte allows for utilization of the same electrolytic species onboth sides of the battery cell, which can reduce electrolytecross-contamination and can increase the efficiency of the IFB system,resulting in less electrolyte replacement as compared to other redoxflow battery systems.

Efficiency losses in an IFB may result from electrolyte crossoverthrough a separator 24 (e.g., ion-exchange membrane barrier,micro-porous membrane, and the like). For example, ferric ions in thepositive electrolyte may be driven toward the negative electrolyte by aferric ion concentration gradient and an electrophoretic force acrossthe separator. Subsequently, ferric ions penetrating the membranebarrier and crossing over to the negative electrode compartment 20 mayresult in coulombic efficiency losses. Ferric ions crossing over fromthe low pH redox side (e.g., more acidic positive electrode compartment22) to high pH plating side (e.g., less acidic negative electrodecompartment 20) may result in precipitation of Fe(OH)₃. Precipitation ofFe(OH)₃ may degrade the separator 24 and cause permanent batteryperformance and efficiency losses. For example, Fe(OH)₃ precipitate maychemically foul the organic functional group of an ion-exchange membraneor physically clog the small micro-pores of an ion-exchange membrane. Ineither case, due to the Fe(OH)₃ precipitate, membrane ohmic resistancemay rise over time and battery performance may degrade. Precipitate maybe removed by washing the battery with acid, but the constantmaintenance and downtime may be disadvantageous for commercial batteryapplications. Furthermore, washing may be dependent on regularpreparation of electrolyte, adding to process cost and complexity.Adding specific organic acids to the positive electrolyte and thenegative electrolyte in response to electrolyte pH changes may alsomitigate precipitate formation during battery charge and dischargecycling.

Additional coulombic efficiency losses may be caused by reduction of H⁺(e.g., protons) and subsequent formation of H₂ (e.g., hydrogen gas) andthe reaction of protons in the negative electrode compartment 20 withelectrons supplied at the plated iron metal electrode to form hydrogengas.

The IFB electrolyte (e.g., FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and the like)is readily available and can be produced at low costs. The IFBelectrolyte offers higher reclamation value because the same electrolytecan be used for the negative electrolyte and the positive electrolyte,consequently reducing cross contamination issues as compared to othersystems. Furthermore, owing to its electron configuration, iron maysolidify into a generally uniform solid structure during plating thereofon the negative electrode substrate. For zinc and other metals commonlyused in hybrid redox batteries, solid dendritic structures may formduring plating. The stable electrode morphology of the IFB system mayincrease the efficiency of the battery in comparison to other redox flowbatteries. Further still, iron redox flow batteries reduce the use oftoxic raw materials and can operate at a relatively neutral pH ascompared to other redox flow battery electrolytes. Accordingly, IFBsystems reduce environmental hazards as compared with all other currentadvanced redox flow battery systems in production.

Continuing with FIG. 1 , a schematic illustration of the redox flowbattery system 10 is shown. The redox flow battery system 10 maycomprise the redox flow battery cell 18 fluidly connected to amulti-chambered electrolyte storage tank 110. The redox flow batterycell 18 may generally include the negative electrode compartment 20,separator 24, and positive electrode compartment 22. The separator 24may comprise an electrically insulating ionic conducting barrier whichprevents bulk mixing of the positive electrolyte and the negativeelectrolyte while allowing conductance of specific ions therethrough.For example, the separator 24 may comprise an ion-exchange membraneand/or a microporous membrane.

In some examples, the separator 24 may include a base membrane materialacting as a microporous framework for the separator 24. The basemembrane may be an ultrahigh molecular weight polyethylene (UHMWPE) thatlends mechanical strength to the separator 24. Silica may beprecipitated onto the UHMWPE to provide porosity and wettability. Theseparator 24 may also include a gel that, when applied to the membranein a solution, may be polymerized and cross-linked within pores of themembrane to form a resin. The resulting resin, derived from thecross-linked polymer gel (CPG), may be adapted to selectively transportions across the membrane via an ion-exchange mechanism. Furthermore, theseparator 24 may be configured to have ribs molded into a surface of theseparator 24 in contact with the negative electrolyte. The ribs may beused in place of a negative spacer that defines a distance between theseparator 24 and the negative electrode 26 as well as flow patterns ofthe negative electrolyte across the negative electrode 26. Descriptionsof a synthetic protocol, identities of functional groups of the CPG, andgeometries of the ribs are provided further below with respect to FIGS.8-12 .

The negative electrode compartment 20 may comprise the negativeelectrode 26 and the negative electrolyte comprising electroactivematerials. The positive electrode compartment 22 may comprise thepositive electrode 28 and the positive electrolyte comprisingelectroactive materials. In some examples, multiple redox flow batterycells 18 may be combined in series or parallel to generate a highervoltage or current in a redox flow battery system. Further illustratedin FIG. 1 are negative and positive electrolyte pumps 30 and 32, bothused to pump electrolyte solution through the flow battery system 10.Electrolytes are stored in one or more tanks external to the cell andare pumped via negative and positive electrolyte pumps 30 and 32 throughthe negative electrode compartment 20 side and the positive electrodecompartment 22 side of the battery, respectively.

The redox flow battery system 10 may also include a first bipolar plate36 and a second bipolar plate 38, each positioned along a rear-facingside, e.g., opposite of a side facing the separator 24, of the negativeelectrode 26 and the positive electrode 28, respectively. The firstbipolar plate 36 may be in contact with the negative electrode 26, andthe second bipolar plate 38 may be in contact with the positiveelectrode 28. In other examples, however, the bipolar plates may bearranged proximate but spaced away from the electrodes. The IFBelectrolytes may be transported to reaction sites at the negative andpositive electrodes 26 and 28 by the first and second bipolar plates 36and 38, resulting from conductive properties of a material of thebipolar plates 36, 38. Electrolyte flow may also be assisted by thenegative and positive electrolyte pumps 30 and 32, facilitating forcedconvection through the redox flow battery cell 18. Reactedelectrochemical species may also be directed away from the reactionsites by the combination of forced convection and the presence of thefirst and second bipolar plates 36 and 38.

The first and second bipolar plates 36 and 38 may each include anelectrically insulated component, which aids in reducing shunt currentsgenerated in the electrolyte, and an electrically conductive component.The electrically conductive component allows the bipolar plates 36, 38to electrically connect cells within a stack of redox flow batterycells. Furthermore, each of the bipolar plates in the cell stack mayhave a positive side and a negative side and, in some examples, may beadapted on the positive side with flow channels for positive electrolyteand flow channels on the negative side for negative electrolyte. Forexample, the cell stack may include more than one of the redox flowbattery cell 18 shown in FIG. 1 , the more than one redox flow batterycells stacked linearly. In such a configuration, the first bipolar plate36 of the redox flow battery cell 18 may be arranged with a negativeside 3 facing the rear side of the negative electrode 26 and a positiveside 5 of the first bipolar plate 36, the positive side opposite of thenegative side, facing a rear side of a positive electrode of an adjacentredox flow battery cell (not shown) positioned on the left-hand side ofredox flow battery cell 18. Similarly, the second bipolar plate 38, witha positive side 7 facing the rear side of the positive electrode 28, mayhave a negative side 9 facing a rear side of a negative electrode ofanother adjacent redox flow battery cell (not shown), the adjacent cellpositioned at the right-hand side of redox flow battery cell 18.

A plurality of bipolar plates within the cell stack may be electricallyconnected to provide a continuous conductive path along the cell stack,channeling current produced from each cell to current collectorsarranged at ends of the cell stack. The first and second bipolar plates36, 38 in redox flow battery cell 18 may have a variety of additionalroles including supporting a cell stack structure, separating positiveelectrolyte and negative electrolyte, and increasing performance of theredox flow battery system 10 by decreasing formation of shunt currentsin the IFB electrolyte. Details of the material from which the bipolarplates may be formed and methods of fabrication are described furtherbelow, with respect to FIGS. 2-7 .

As illustrated in FIG. 1 , the redox flow battery cell 18 may furtherinclude negative battery terminal 40 and positive battery terminal 42.When a charge current is applied to the battery terminals 40 and 42, thepositive electrolyte is oxidized (loses one or more electrons) at thepositive electrode 28, and the negative electrolyte is reduced (gainsone or more electrons) at the negative electrode 26. During batterydischarge, reverse redox reactions occur on the electrodes. In otherwords, the positive electrolyte is reduced (gains one or more electrons)at the positive electrode 28, and the negative electrolyte is oxidized(loses one or more electrons) at the negative electrode 26. Theelectrical potential difference across the battery is maintained by theelectrochemical redox reactions in the positive electrode compartment 22and the negative electrode compartment 20 and may induce a currentthrough a conductor while the reactions are sustained. The amount ofenergy stored by a redox battery is limited by the amount ofelectro-active material available in electrolytes for discharge,depending on the total volume of electrolytes and the solubility of theelectro-active materials.

The flow battery system 10 may further comprise the integratedmulti-chambered electrolyte storage tank 110. The multi-chamberedstorage tank 110 may be divided by a bulkhead 98. The bulkhead 98 maycreate multiple chambers within the storage tank so that both thepositive and negative electrolyte may be included within a single tank.The negative electrolyte chamber holds negative electrolyte comprisingelectroactive materials, and the positive electrolyte chamber 52 holdspositive electrolyte comprising electroactive materials. The bulkhead 98may be positioned within the multi-chambered storage tank 110 to yield adesired volume ratio between the negative electrolyte chamber 50 and thepositive electrolyte chamber 52. In one example, the bulkhead 98 may bepositioned to set the volume ratio of the negative and positiveelectrolyte chambers according to the stoichiometric ratio between thenegative and positive redox reactions. The figure further illustratesthe fill height 112 of storage tank 110, which may indicate the liquidlevel in each tank compartment. The figure also shows gas head space 90located above the fill height 112 of negative electrolyte chamber 50 andgas head space 92 located above the fill height 112 of positiveelectrolyte chamber 52. The gas head space 92 may be utilized to storehydrogen gas generated through operation of the redox flow battery(e.g., due to proton reduction and corrosion side reactions) andconveyed to the multi-chambered storage tank 110 with returningelectrolyte from the redox flow battery cell 18. The hydrogen gas may beseparated spontaneously at the gas-liquid interface (e.g., fill height112) within the multi-chambered storage tank 110, thereby precludinghaving additional gas-liquid separators as part of the redox flowbattery system. Once separated from the electrolyte, the hydrogen gasmay fill the gas head spaces 90 and 92. A such, the stored hydrogen gasmay aid in purging other gases from the multi-chamber storage tank 100,thereby acting as an inert gas blanket for reducing oxidation ofelectrolyte species, which may help to reduce redox flow batterycapacity losses. In this way, utilizing the integrated multi-chamberedstorage tank 110 may forego having separate negative and positiveelectrolyte storage tanks, hydrogen storage tanks, and gas-liquidseparators common to conventional redox flow battery systems, therebysimplifying the system, reducing the physical footprint of the system,and reducing system costs.

FIG. 1 also shows the spill-over hole 96, which creates an opening inthe bulkhead 98 between gas head spaces 90 and 92, and provides a meansof equalizing gas pressure between the two chambers. The spill-over hole96 may be positioned a threshold height above the fill height 112. Thespill-over hole 96 further enables a capability to self-balance theelectrolytes in each of the positive and negative electrolyte chambersin the event of a battery crossover. In the case of an all iron redoxflow battery system, the same electrolyte (Fe²⁺) is used in bothnegative and positive electrode compartments 20 and 22, so spilling overof electrolyte between the negative and positive electrolyte chambers 50and 52 may reduce overall system efficiency, but the overall electrolytecomposition, battery module performance, and battery module capacity aremaintained. Flange fittings may be utilized for all piping connectionsfor inlets and outlets to and from the multi-chambered storage tank 110to maintain a continuously pressurized state without leaks. Themulti-chambered storage tank can include at least one outlet from eachof the negative and positive electrolyte chambers and at least one inletto each of the negative and positive electrolyte chambers. Furthermore,one or more outlet connections may be provided from the gas head spaces90 and 92 for directing hydrogen gas to rebalancing reactors 80 and 82.

Although not shown in FIG. 1 , integrated multi-chambered electrolytestorage tank 110 may further include one or more heaters thermallycoupled to each of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52. In alternate examples, only one of the negativeand positive electrolyte chambers may include one or more heaters. Inthe case where only the positive electrolyte chamber 52 includes one ormore heaters, the negative electrolyte may be heated by transferringheat generated at the battery cells of the power module to the negativeelectrolyte. In this way, the battery cells of the power module may heatand facilitate temperature regulation of the negative electrolyte. Theone or more heaters may be actuated by the controller 88 to regulate atemperature of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52 independently or together. For example, inresponse to an electrolyte temperature decreasing below a thresholdtemperature, the controller may increase a power supplied to one or moreheaters so that a heat flux to the electrolyte is increased. Theelectrolyte temperature may be indicated by one or more temperaturesensors mounted at the multi-chambered electrolyte storage tank 110,including sensors 60 and 62. As examples, the one or more heaters mayinclude coil type heaters or other immersion heaters immersed in theelectrolyte fluid, or surface mantle type heaters that transfer heatconductively through the walls of the negative and positive electrolytechambers to heat the fluid therein. Other known types of tank heatersmay be employed without departing from the scope of the presentdisclosure. Furthermore, controller 88 may deactivate one or moreheaters in the negative and positive electrolyte chambers 50, 52 inresponse to a liquid level decreasing below a solids fill thresholdlevel. Said in another way, controller 88 may activate the one or moreheaters in the negative and positive electrolyte chambers 50, 52 only inresponse to a liquid level increasing above the solids fill thresholdlevel. In this way, activating the one or more heaters withoutsufficient liquid in the positive and/or negative electrolyte chamberscan be averted, thereby reducing a risk of overheating or burning outthe heaters.

Further still, one or more inlet connections may be provided to each ofthe negative and positive electrolyte chambers 50, 52 from a fieldhydration system (not shown). In this way, the field hydration systemcan facilitate commissioning of the redox flow battery system, includinginstalling, filling, and hydrating the system, at an end-use location.Furthermore, prior to its commissioning at the end-use location, theredox flow battery system may be dry-assembled at a batterymanufacturing facility different from end-use location without fillingand hydrating the system, before delivering the system to the end-uselocation. In one example, the end-use location may correspond to thelocation where the redox flow battery system 10 is to be installed andutilized for on-site energy storage. Said in another way, it isanticipated that, once installed and hydrated at the end-use location, aposition of the redox flow battery system 10 becomes fixed, and theredox flow battery system 10 is no longer deemed a portable, dry system.Thus, from the perspective of a redox flow battery system end-user, thedry portable redox flow battery system 10 may be delivered on-site,after which the redox flow battery system 10 is installed, hydrated andcommissioned. Prior to hydration the redox flow battery system 10 may bereferred to as a dry, portable system, the redox flow battery system 10being free of or without water and wet electrolyte. Once hydrated, theredox flow battery system 10 may be referred to as a wet non-portablesystem, the redox flow battery system 10 including wet electrolyte.

Further illustrated in FIG. 1 , electrolyte solutions typically storedin the multi-chambered storage tank 110 are pumped via negative andpositive electrolyte pumps 30 and 32 throughout the flow battery system10. Electrolyte stored in negative electrolyte chamber 50 is pumped vianegative electrolyte pump 30 through the negative electrode compartment20 side, and electrolyte stored in positive electrolyte chamber 52 ispumped via positive electrolyte pump 32 through the positive electrodecompartment 22 side of the battery.

Two electrolyte rebalancing reactors 80 and 82 may be connected in-lineor in parallel with the recirculating flow paths of the electrolyte atthe negative and positive sides of the battery cell 18, respectively, inthe redox flow battery system 10. One or more rebalancing reactors maybe connected in-line with the recirculating flow paths of theelectrolyte at the negative and positive sides of the battery, and otherrebalancing reactors may be connected in parallel for redundancy (e.g.,a rebalancing reactor may be serviced without disrupting battery andrebalancing operations) and for increased rebalancing capacity. In oneexample, the electrolyte rebalancing reactors 80 and 82 may be placed inthe return flow path from the positive and negative electrodecompartments 20 and 22 to the positive and negative electrolyte chambers50 and 52, respectively. Electrolyte rebalancing reactors 80 and 82 mayserve to rebalance electrolyte charge imbalances in the redox flowbattery system occurring due to side reactions, ion crossover, and thelike, as described herein. In one example, electrolyte rebalancingreactors 80 and 82 may include trickle bed reactors, where the hydrogengas and electrolyte are contacted at catalyst surfaces in a packed bedfor carrying out the electrolyte rebalancing reaction. In other examplesthe rebalancing reactors 80 and 82 may include flow-through typereactors that are capable of contacting the hydrogen gas and theelectrolyte liquid and carrying out the rebalancing reactions in theabsence of a packed catalyst bed.

During operation of the redox flow battery system 10, sensors and probesmay monitor and control chemical properties of the electrolyte such aselectrolyte pH, concentration, state of charge, and the like. Forexample, as illustrated in FIG. 1 , sensors 62 and 60 may be positionedto monitor positive electrolyte and negative electrolyte conditions atthe positive electrolyte chamber 52 and the negative electrolyte chamber50, respectively. In another example, sensors 62 and 60 may each includeone or more electrolyte level sensors to indicate a level of electrolytein the positive electrolyte chamber 52 and the negative electrolytechamber 50, respectively. As another example, sensors 72 and 70, alsoillustrated in FIG. 1 , may monitor positive electrolyte and negativeelectrolyte conditions at the positive electrode compartment 22 and thenegative electrode compartment 20, respectively. Sensors may bepositioned at other locations throughout the redox flow battery system10 to monitor electrolyte chemical properties and other properties. Forexample, a sensor may be positioned in an external acid tank (not shown)to monitor acid volume or pH of the external acid tank, wherein acidfrom the external acid tank is supplied via an external pump (not shown)to the redox flow battery system 10 in order to reduce precipitateformation in the electrolytes. Additional external tanks and sensors maybe installed for supplying other additives to the redox flow batterysystem 10. For example, various sensors, including temperature,conductivity, and level sensors of a field hydration system may transmitsignals to the controller 88. Furthermore, controller 88 may sendsignals to actuators such as valves and pumps of the field hydrationsystem during hydration of the redox flow battery system 10. Sensorinformation may be transmitted to controller 88, which may in turnactuate pumps 30 and 32 to control electrolyte flow through the cell 18or to perform other control functions, as an example. In this manner,the controller 88 may be responsive to one or a combination of sensorsand probes.

Redox flow battery system 10 may further comprise a source of hydrogengas. In one example, the source of hydrogen gas may comprise a separatededicated hydrogen gas storage tank. In the example of FIG. 1 , hydrogengas may be stored in and supplied from the integrated multi-chamberedelectrolyte storage tank 110. Integrated multi-chambered electrolytestorage tank 110 may supply additional hydrogen gas to the positiveelectrolyte chamber 52 and the negative electrolyte chamber 50.Integrated multi-chambered electrolyte storage tank 110 may alternatelysupply additional hydrogen gas to the inlet of electrolyte rebalancingreactors 80 and 82. As an example, a mass flow meter or other flowcontrolling device (which may be controlled by controller 88) mayregulate the flow of the hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110. The integrated multi-chambered electrolytestorage tank 110 may supplement the hydrogen gas generated in redox flowbattery system 10. For example, when gas leaks are detected in redoxflow battery system 10 or when the reduction reaction rate is too low atlow hydrogen partial pressure, hydrogen gas may be supplied from theintegrated multi-chambered electrolyte storage tank 110 in order torebalance the state of charge of the electro-active species in thepositive electrolyte and negative electrolyte. As an example, controller88 may supply hydrogen gas from integrated multi-chambered electrolytestorage tank 110 in response to a measured change in pH or in responseto a measured change in state of charge of an electrolyte or anelectro-active species. For example, an increase in pH of the negativeelectrolyte chamber 50, or the negative electrode compartment 20, mayindicate that hydrogen is leaking from the redox flow battery system 10and/or that the reaction rate is too slow with the available hydrogenpartial pressure, and controller 88, in response to the pH increase, mayincrease a supply of hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110 to the redox flow battery system 10. As afurther example, controller 88 may supply hydrogen gas from integratedmulti-chambered electrolyte storage tank 110 in response to a pH change,wherein the pH increases beyond a first threshold pH or decreases beyondsecond threshold pH. In the case of an IFB, controller 88 may supplyadditional hydrogen to increase the rate of reduction of ferric ions andthe rate of production of protons, thereby reducing the pH of thepositive electrolyte. Furthermore, the negative electrolyte pH may belowered by hydrogen reduction of ferric ions crossing over from thepositive electrolyte to the negative electrolyte or by proton generatedat the positive side crossing over to the negative electrolyte due to aproton concentration gradient and electrophoretic forces. In thismanner, the pH of the negative electrolyte may be maintained within astable region while reducing the risk of precipitation of ferric ions(crossing over from the positive electrode compartment) as Fe(OH)₃.Other control schemes for controlling the supply rate of hydrogen gasfrom integrated multi-chambered electrolyte storage tank 110 responsiveto a change in an electrolyte pH or to a change in an electrolyte stateof charge, detected by other sensors such as an oxygen-reductionpotential (ORP) meter or an optical sensor, may be implemented. Furtherstill, the change in pH or state of charge triggering the action ofcontroller 88 may be based on a rate of change or a change measured overa time period. The time period for the rate of change may bepredetermined or adjusted based on the time constants for the redox flowbattery system 10. For example the time period may be reduced if therecirculation rate is high, and local changes in concentration (e.g.,due to side reactions or gas leaks) may quickly be measured since thetime constants may be small.

An IFB system, e.g., the flow battery system 10 of FIG. 1 , may includea power module adapted with a bipolar plate adapted with an integratednegative electrode, described further below with reference to FIGS. 2,3, and 5-7 . A negative spacer, defining a spacing between the negativeelectrode and adjacent components and moderating flow paths along thenegative electrode, may be positioned between the negative electrode andthe adjacent components. In some examples, the negative spacer may bedirectly molded, as shown in FIGS. 9 and 10 , into a surface of amembrane separator that separates positive electrolyte from negativeelectrolyte. An example of a power module 1200 that may be used in aredox flow battery system, such as the redox flow battery system 10 ofFIG. 1 , is shown in FIG. 12 . A set of reference axis 1201 is provided,indicating a y-axis, an x-axis, and a z-axis. The power module 1200comprises a series of components arranged as layers within the powermodule 1200. The layers may be positioned co-planar with the y-x planeand stacked along the z-axis.

Pressure plates 1202 may be arranged at a first end 1203 and a secondend 1205 of the power module 1200 that provide rigid end walls thatdefine boundaries of the power module 1200. The pressure plates 1202allow layers of the power module 1200 to be pressed together between thepressure plates 1202 to seal components of the power module within aninterior 1207 of the power module 1200. Picture frames 1204 may bearranged inside of the pressure plates 1202, e.g., against a side facingthe interior 1207 of each of the pressure plates of the power module1200, the picture frames 1204 adapted to interface with one another toseal fluids within the interior 1207 of the power module 1200.

Elements of the power module 1200 are now described along a directionfrom the first end 1203 towards the second end 1205. A negative spacer1206 is arranged adjacent to one of the picture frames that is proximateto the first end 1203, the negative spacer 1206 defining flow channelsalong a surface of a negative electrode that may be integrated into asurface of a bipolar plate 1208 that is facing the negative spacer 1206.In some examples, the negative spacer 1206 may be directly molded into asurface of a membrane separator 1214 as a plurality of ribs along asurface of the membrane 1214 that is in face-sharing contact with thenegative electrode and on an opposite side of the membrane separator1214 from a positive electrode 416.

The bipolar plate 1208 may be the first or second bipolar plate 36 or 38of FIG. 1 and, in one example, may have the negative electrodeintegrated into the surface of the bipolar plate 1208 in face-sharingcontact with the negative spacer 1206. The bipolar plate 1208 ispositioned between the negative spacer 1206 and surrounded by a bipolarplate frame plate 1210 that provides structural support.

The positive electrode 1212 is arranged adjacent to a side of thebipolar plate 1208 that is opposite of the negative spacer 1206. Themembrane separator 1214, surrounded by a membrane frame plate 1216 forstructural support, may be positioned adjacent to the positive electrode1212, towards the second end 1205 of the power module 1200. Thecomponents described above, e.g., the negative spacer 1206, the bipolarplate 1208, the positive electrode 1212, and the membrane separator 1214may repeat within the power module, from the first end 1203 to thesecond end 1205, a number of times, thereby forming a battery stack.Negative electrolyte may be contained between another membrane separatorthat is arranged on the side of the bipolar plate 1208 facing the firstend 1203 of the power module 1200, with the negative electrolyte incontact with both the negative spacer 1206 and integrated negativeelectrode (e.g., integrated into the surface of the bipolar plate 1208).Positive electrolyte may be contained between the bipolar plate 1208 andthe membrane separator 1214, in contact with the positive electrode1212.

Within a power module, a bipolar plate may be arranged between anegative electrode and a positive electrode and configured to interactwith both negative and positive electrolytes. A membrane separator,e.g., the separator 24 of FIG. 1 and the membrane separator 1214 of FIG.12 , may be also be included in the stack, similarly positioned betweenthe positive and negative electrode on opposite sides of the positiveand negative electrode from the bipolar plate. Both the bipolar plateand membrane separator may be repeating components in the IFB systemthat contribute significantly to system costs. By manufacturing thebipolar plate and membrane separator by cost-efficient, automatedprocesses, production expenses may be greatly reduced.

In one example, the bipolar plate may be integrated with the negativeand positive electrodes into a unified, continuous structure that may bemanufactured by a roll-to-roll (R2R) process. As such, the bipolar platehas a negative side that is adapted for iron plating according toequation (1) above and a positive side that engages with positiveelectrolyte to facilitate the reaction of equation (2). Fabricating thebipolar plate with the negative and positive electrodes incorporatedinto the structure may simplify the production of IFB components andreduce costs significantly. In addition, the R2R process for fabricatingthe bipolar plate does not involve curing of materials, therebydecreasing processing time as compared to molding methods. Furthermore,using the R2R process allows for continuous, large-scale production ofthe bipolar plate using a commercially available carbon fiber as a basewhich may be optimized for cohesion with materials forming the negativeand position electrodes.

An example of a R2R system 700 that may be used to manufacture a bipolarplate is shown in a schematic diagram in FIG. 7 . A reference axis 702is provided, indicating a y-axis, an x-axis, and a z-axis. In oneexample, the y-axis may be aligned with a vertical direction, the x-axiswith a horizontal direction, and the z-axis with a lateral direction.The R2R system 700 may include a first roller 704 at a first end 703 ofthe R2R system 700 and a second roller 706 at a second end 705 of theR2R system 700, the first roller 704 and the second roller 706 spacedapart and aligned along the horizontal direction. The first roller 704is a support for a flexible substrate 708, the flexible substrate 708rolled around the first roller 704 and extending horizontally from thefirst roller 704 to the second roller 706. The substrate 708 may bemoving, with the first roller 704 and second roller 706 rotatingclockwise, across a distance 710 from the first roller 704 towards thesecond roller 706, as indicated by arrows 712. As the substrate 708travels, the substrate 708 is unrolled from the first roller 704 andreeled onto the second roller 706.

As the substrate 708 travels across the distance 710, a series ofprocesses may be applied to the substrate 708. For example, at firststep A, a rolled sheet of a material 714 may be unreeled and added to atop surface, with respect to the y-axis, of the substrate 708. Thematerial 714 may form an upper layer 716 of a final product 720 of theR2R system 700, adapted with desired properties such as, for example,electrical conductivity. Additionally processing steps may occur along apath of travel of the substrate 708. As one example, a substance may besprayed or printed onto the top surface of the substrate 708 by a firstdevice 730 at a second step B. At a third step C, the substrate 708 maybe pressed or heated to bond the substrate 708 to the upper layer 716 ata second device 732. Furthermore, similar treatments may be applied to abottom surface (not shown in FIG. 7 ), with respect to the y-axis, ofthe substrate 708 or a roll of another material may be unreeled andcoupled to the bottom surface of the substrate 708. It will beappreciated that the first, second and third steps A, B, and C, shown inFIG. 7 and associated processes described above are non-limitingexamples of treatment steps that may be included in the R2R system 700.Various combinations of steps, other treatment processes not includedabove, and number of processing steps may be applied without departingfrom the scope of the present disclosure.

The R2R system may also include a cutting tool 718 along the distance710 between the first roller 704 and second roller 706 and positionedproximate to the second roller 706. The cutting tool 718 may be appliedto the substrate 708 and bonded upper layer 716 (together forming theR2R product 720), as well as any other layers added to the substrate708, to cut through the R2R product 720. The R2R product 720, may thenbe stored as a roll on the second roller 706 and unrolled and cut todesired geometries.

A first example of a R2R process for manufacturing a continuous, carbonfiber-reinforced composite bipolar plate 205 is depicted in a flowdiagram 200 in FIG. 2 . A set of references axes 201 are provided inFIGS. 2 and 3 , indicating a y-axis, an x-axis, and a z-axis. The R2Rprocess may be conducted on a R2R system, such as the R2R system 700 ofFIG. 7 . As one example, the bipolar plate 205 may be assembled and usedin an IFB such as the redox flow battery cell 18 of FIG. 1 . At a firststep 202 in the flow diagram 200, a carbon fiber sheet 204 may be anon-limiting example of the flexible substrate 708 of FIG. 7 and used asa base layer. The carbon fiber sheet 204 may be a commerciallyavailable, off-the-shelf substrate imbedded with resin, the resinallowing the carbon fiber sheet 204 to act as an electrolyte-impermeablebarrier. The resin may provide an electrically insulating component ofthe substrate 708 while the carbon fiber provides electricallyconductive channels through the substrate 708. The carbon fiber sheet204 may be prepregnated with high surface area carbon particles at asecond step 206.

In other examples, the carbon fiber sheet 204 may instead be animpermeable metal sheet, providing both impermeability to electrolyteflow and conductivity. The metal sheet may be formed from a thin sheetof titanium, iron, stainless steel, or a precious metal. Alternatively,the metal sheet may be formed from a mesh of any of the metals listedabove and filled with the resin to block electrolyte flow therethrough.By fabricating the metal sheet from a thin sheet of a metallic materialor a metallic mesh, electrical conductivity and structural support isafforded to the bipolar plate 205 with reduced cost compared to theresin-imbedded carbon fiber sheet 204.

Prepregnation of the carbon fiber sheet 204 (or metallic mesh) isachieved by coating an upper face 203 of the carbon fiber sheet 204 witha layer of carbon particles to form a carbon powder coat 208. The carbonpowder coat 208 is a negative electrode, e.g., the negative electrode 26of FIG. 1 , acting as a surface for Fe⁰ plating during charging of theIFB. As such, the carbon powder coat 208 provides a surface structure toa negative side of the bipolar plate 205 that is conducive to depositionof iron metal onto the carbon powder coat 208.

The carbon powder coat 208 may be applied to the upper face 203 of thecarbon fiber sheet 204 by various methods. For example, the high surfacearea carbon particles may be suspended in an organic solvent such asmethyl ethyl ketone to form an ink with a carbon concentration of 2 wt%. The ink may be spray coated, doctor-bladed, or screen printed ontothe carbon fiber sheet 204 to form the carbon powder coat 208 with anoverall carbon content of 1 to 15 wt %.

At a third step 212 of the flow diagram 200, the carbon fiber sheet 204with the carbon powder coat 208 may be coupled to graphite felt 214along a bottom face 216 of the carbon fiber sheet 204, the bottom face216 opposite of the upper face 203. In other examples, the graphite felt214 may instead be carbon felt. The graphite felt 214 is a positiveelectrode, interacting with positive electrolyte to facilitate the redoxreaction of equation (1). The graphite felt 214 may be thermally bondedto the bottom face 216 of the carbon fiber sheet 204 by heating thestructure so that the resin imbedded in the carbon fiber sheet 204melts. Upon melting, an upper face 218 of the graphite felt 214 isdirectly attached to the bottom face 216 of the carbon fiber sheet 204.The bipolar plate 205 is thus produced with the carbon fiber sheet 204sandwiched between the negative electrode, formed from the carbon powdercoat 208, and the positive electrode, formed from the graphite felt 214.The resin of the resin-imbedded carbon fiber sheet 204 may provide anon-conductive component of the bipolar plate 205 while the negative andpositive electrodes provide conductive components.

A second example of a R2R process for manufacturing a bipolar plate 305is illustrated in a flow diagram 300 in FIG. 3 where the bipolar plate305, once assembled, may also be included in an IFB system. The R2Rsystem may also be conducted on a R2R system such as the R2R system 700of FIG. 7 . The flow diagram 300 shows a first step 302 that includesstacking three layers of flexible materials which may each becommercially available: a top layer of carbon paper 304, a middle layerof thermoplastic 306 that may be the flexible substrate 708 of FIG. 7 ,and a bottom layer of felt 308 that may be graphite- or carbon-based.The layers may be arranged so that a bottom face 310 of the carbon paper304 is in face-sharing contact with a top face 312 of the thermoplastic306 and a bottom face 314 of the thermoplastic 306 is in face-sharingcontact with a top face 316 of the felt 308.

In the R2R process of FIG. 3 , the thermoplastic 306 may be aninsulating, impervious liquid barrier, similar to the resin imbeddedcarbon fiber sheet 204 of FIG. 2 . The carbon paper 304 may be anegative electrode of the bipolar plate 305 and the felt 308 may be apositive electrode, analogous to the carbon powder coat 208 and graphitefelt 214 of FIG. 2 , respectively. The three layers of the bipolar plate305 may be sewn together using a thread 318 as shown at a second step320 of the flow diagram 300.

At the second step 320, the thread 318, formed from a conductivematerial such as carbon or graphite, may maintain the three stackedlayers coupled together and also provide a conductive path through thethermoplastic 306. Electron flow is thus enabled across the otherwisenon-conductive thermoplastic 306. The insertion of the thread 318through a thickness 322 of the bipolar plate 305 may be achieved byvarious sewing methods, including a needle punch with 1 k-10 k tow.Stitches 324 of the thread 318 may form loops through at least a portionof the thickness 322, extending from a top face 326 of the carbon paper304 towards a bottom face 328 of the felt 308. As shown in FIG. 3 , thestitches 324 may not pierce through the bottom face 328 of the felt 308but in other examples, each stitch of the stitches 324 may extendentirely through a thickness, defined along the y-axis, of the felt 308,exiting the felt 308 at the bottom face 328 and re-entering the bottomface 328 in a continuous and sinuous manner.

The penetration of the thread 318 through the thermoplastic 306 mayresult in a puncturing of the thermoplastic 306, creating a plurality ofapertures, or through-holes, extending through an entire depth 330 ofthe thermoplastic 306. A presence of through-holes may allow leakage ofboth positive and negative electrolytes across the thermoplastic 306. Tomitigate electrolyte leakage, the bipolar plate 305 may be heat-pressedat a third step 332 of the flow diagram 300. By heat-pressing thebipolar plate 305, the thermoplastic 306 may soften and deformsufficiently to close and seal the through-holes, allowing thethermoplastic 306 to provide an electrolyte-impermeable layer betweenthe negative and positive electrodes.

Both the processes depicted in the first and second flow diagrams 200,300 for manufacturing a bipolar plate may be adapted as R2R processeswhere a flexible substrate, such as the resin imbedded carbon fibersheet or metallic sheet 204 of FIG. 2 and the thermoplastic 306 of FIG.3 may be positioned between a first material (e.g., the carbon powdercoat 208 of FIG. 2 or the carbon paper 304 of FIG. 3 ) which may becoated or rolled onto a first side of the substrate. A second material(e.g., the carbon or graphite felt 214 and 308 of FIGS. 2 and 3 ) may besimultaneously rolled onto a second side of the substrate. The R2Rprocesses allow larger areas of material to be produced in a shorterperiod of time in comparison to batch processes such as injection andcompression molding. The method may be further simplified by relying oncommercially available materials, as described above, thereby reducing anumber of processing steps involved.

A prophetic graph 400 comparing manufacturing costs of a currentstate-of-the-art compression molding process versus a R2R process, suchas the processes illustrated in the flow diagrams 200 and 300 of FIGS. 2and 3 , is shown in FIG. 4 . A measure of a number of bipolar platesproduced is given along an x-axis of the graph 400 and a unit cost perbipolar plate is provided along a y-axis. Plot 402 shows a relationshipbetween the number of bipolar plates produced and the unit cost for thecompression molding process. Plot 404 shows a relationship between thenumber of bipolar plates produced and the unit cost for the R2R process.

At an onset of production, e.g., left-hand side of graph 400, an initialunit cost for plot 402 is nearly double that of plot 404. As the numberof bipolar plates increases, both plot 402 and 404 decrease sharply.Between 1,000,000 and 10,000,000 pieces, plot 404 remains relativelyuniform, decreasing slightly per bipolar plate. Plot 402, however, showsa greater decrease, from from 1,000,000 to 10,000,000 pieces. At10,000,000, in spite of the compression molding process becoming morecost efficient, the unit price for bipolar plate production is stilldouble that of the R2R process. The information provided in graph 400indicates that regardless of quantity, the R2R process results insignificant savings with regards to manufacturing costs.

A first example of a method 500 for fabricating a bipolar plate via aR2R process is shown in FIG. 5 . The method may be similar to theprocess illustrated in the flow diagram 200 of FIG. 2 , allowingscaled-up production of the bipolar plate in a cost-effective manner,and performed on an R2R system, such as the R2R system 700 of FIG. 7 .The R2R process may include a roll of a substrate as a starting materialthat is unrolled from a first roller, e.g., the first roller 704 of FIG.7 , of the R2R system and arranged so that the substrate extends fromthe first roller to a second roller, e.g., the second roller 706 of FIG.7 , at a second end of the R2R system. The substrate is processed as thesubstrate is moved from the first roller to a second roller. The finalproduct may be reeled onto the second roller and distributed as a rolledquantity of the product, e.g., the bipolar plate.

At 502, the method includes arranging a first roll storing a substratealong a first end of the R2R system. The substrate may be a flexiblematerial such as carbon fiber, as one example, imbedded with resin thatallows the substrate to be a non-conductive layer impermeable to fluids.As another example, the substrate may be a metal sheet or a metal meshwith a resin filler. As the substrate is guided from the first roller tothe second roller, the substrate is unrolled and positioned so that thesubstrate extends from the first roller to the second roller. Thesubstrate is continuously unrolled as the second and/or first roller isrotated. As the substrate is unrolled and moving, a first surface, e.g.,an upper surface, of the substrate is coated with a material for anegative electrode at 504.

Coating the first surface of the substrate includes applying a layer ofhigh surface area carbon particles. The carbon particles may besuspended in a solvent as an ink and spray coated, doctor-bladed, orscreen printed onto the first surface of the substrate, thereby formingthe negative electrode as a layer on top of the substrate. The surfaceof the negative electrode, comprising a coat of the high surface carbonparticles, provides a surface on which Fe⁰ may readily plate.

The method includes, at 506, bonding a second (or bottom) surface,opposite of the first surface, of the substrate to a material for apositive electrode while the substrate is travelling from the firstroller to the second roller. The material for the positive electrode, asone example, may be a graphite or carbon felt that assists infacilitating oxidation of iron when in contact with a positiveelectrolyte during charging of the IFB and reduction of iron when theIFB is discharging. The felt may be arranged in a roll that is fed alonga direction from the first roller to the second roller and onto thesecond surface of the substrate as the substrate is moving so that thefelt is in face-sharing contact with the bottom of the substrate.Bonding of the felt to the substrate may be achieved by heat-pressingthe substrate, with the negative electrode coupled to the first surfaceof the substrate, and the felt coupled to the second surface. Theapplied heat melts the resin imbedded in the substrate and as thesubstrate and felt are pressed together, the melted resin adheres to thefelt. The felt forms the positive electrode as a layer under the secondsurface of the substrate, on an opposite face from the negativeelectrode and the resin, once cooled, maintains the impermeability ofthe substrate layer within the bipolar plate. At 508, the assembledbipolar plate is collected as a roll at a second end of the R2R systemon the second roller.

A second example of a method 600 for fabricating a bipolar plate via aR2R process is shown in FIG. 6 which may also be conducted on an R2Rsystem, such as the R2R system 700 of FIG. 7 . The method may be similarto the process illustrated in the flow diagram 300 of FIG. 3 , allowingscaled-up production of the bipolar plate in a cost-effective manner.The R2R process may include simultaneously feeding rolls of materialsfrom a first roller at a first end of the R2R system towards a secondroller at a second end. Processing steps may be conducted as thematerials are guided across the R2R system. The final product may bereeled onto the second roller, cut, and distributed as a rolled quantityof the product, e.g., the bipolar plate.

At 602, the method includes arranging rolls of a first, second, andthird material along the first end of the R2R system. The first materialmay be a top layer that forms a negative electrode. For example, thefirst material may be carbon paper that provides a suitable surface forplating of iron metal during charging of the IFB. The second material,forming a middle layer, may be a thermoplastic that is non-conductiveand impermeable to fluids. The third material may be a bottom layer thatforms a positive electrode. The third material may be a carbon orgraphite felt assisting in facilitating oxidation of iron when incontact with a positive electrolyte during charging of the IFB ischarging and reduction of iron during discharging of the IFB. The rollsmay be positioned according to a desired order of stacking of layers,e.g., the roll of the first material is above the roll of the secondmaterial and the roll of the third material is below the roll of thesecond material. The second material may unrolled and guided so that thesecond material extends across the first and second rollers.

At 604 the second material is steered towards the second roller and thesecond end of the R2R system by rotation of the second roller and/orfirst roller. As the second material is moving, the first material isunrolled and guided onto an upper surface of the second material and thethird material is simultaneously unrolled and guided onto a lowersurface of the second material, resulting in a stacked configuration ofthe three materials. At 606, the method includes sewing the layers ofthe bipolar plate assembly while the stacked materials are moving fromthe first roller to the second roller so that the layers are coupled andsecured to one another, forming a unitary sheet. The layers may be sewnusing a conductive thread, such as, for example, a carbon or graphitethread to enable transmission of electron flow through thethermoplastic. The thread may pierce through a top face of the toplayer, through the middle layer of thermoplastic, and penetrate throughat least a portion of a thickness of the bottom layer so that stitchesformed by the thread do not puncture through a bottom face of the bottomlayer of the bipolar plate. The thread may be inserted into the bipolarplate so that the stitches each form a loop within a thickness of thebipolar plate, as shown by the thread 318 of FIG. 3 .

The stacked layers of the bipolar plate assembly are hot-pressedtogether at 608. The applied heat induces melting of the thermoplasticso that the thermoplastic seals around apertures extending through athickness of the thermoplastic created by penetration of the thread. Theimpermeability of the thermoplastic to fluids is thus maintained. Inaddition, melting and subsequent solidification of the thermoplasticwhile pressed against the top and bottom layers, bonds the firstmaterial to an upper surface of the thermoplastic, forming the negativeelectrode, and bonds the third material to a lower surface of thethermoplastic, forming the positive electrode. The assembled bipolarplate, with the thermoplastic sandwiched between the negative andpositive electrodes, is collected onto the second roller at the secondend of the R2R system and cut so that the rolled bipolar plate may betransported and distributed.

In addition to a bipolar plate, a power module of an IFB may alsoinclude a membrane separator, e.g., separator 24 of FIG. 1 , arranged ina battery cell, such as the battery cell 18 of FIG. 1, between anegative electrolyte compartment and a positive electrolyte compartment.The separator may be a barrier to free exchange of electrolyteconstituents between the negative and positive sides of the cell. Morespecifically, in an IFB, the separator blocks flow of Fe³⁺ and FeCl₃from the positive side of the cell to the negative side where Fe²⁺ mayacquire electrons, resulting in reduction of Fe²⁺ to Fe²⁺, anddecreasing an efficiency of the IFB. Other ions, such as H⁺ and K⁺, maybe exchanged across the separator to maintain a chemical and chargebalance across the cell.

The separator may have a combination of UHMWPE and silica as a basemembrane that is coated with an ionomer, such as perfluorosulfonate. Insuch separators, the ionomer coating may be expensive and drive up anoverall cost of the IFB. In order to decrease costs, a differentapproach enabling ion selectivity of the separator may be desirable.

One alternative may include depositing a cross-linked polymer networkwithin pores of the membrane separator from a cross-linked polymer gel(CPG). The CPG may include functional groups, such as sulfonic acid oramine groups, which may interact with cationic constituents inelectrolyte. Examples of monomers that may be used as building blocksfor the polymer network are shown in FIG. 8 .

A first monomer 802 that may be used in a CPG to form a polymer networkin pores of a membrane separator of an IFB may be an acrylamide monomer.The first monomer 802 may have an amide moiety and an ion selectivity ofthe first monomer 802 may rely on an electron-donating property of thenitrogen to enable cross-linking and polymerization. Carbonyl groups onthe polymer may interact with targeted ionic species in an electrolyte.The interaction transports the ionic species across the membrane by ionexchange, thus allowing certain ionic species to flow between sides ofthe separator while inhibiting flow of other species.

Another example of a monomer that may provide ion selectivity in theseparator is shown by a second monomer 804. The second monomer 804 maybe 2-acrylamido-2-methylpropane sulfonic acid (AMPS). A backbone of apolymer formed by AMPS includes carbon, nitrogen, and sulfur-basedfunctional groups, imparting the polymer network formed by the secondmonomer 804 with an ion selectivity that may be different from an ionselectivity of a polymer network based on the first monomer 802.Similarly, a third monomer 806, shown as a sodium 4-vinyl benzenesulfonate salt, may form a polymer network with different electronicproperties than the first monomer 802 or second monomer 804 due to apresence of benzene rings in a polymer framework formed from the thirdmonomer 806. Additionally volumes occupied by each type of cross-linkedpolymer network based on the first, second and third monomers 802, 804,and 806, may vary due to differences in rigidity and molecular size ofeach monomer. For example, the first monomer 802, with a shorter chainof carbon and nitrogen than either the second monomer 804 or the thirdmonomer 806 may be suited to a membrane with small pore size relative toa membrane to which the much longer second monomer 804 may be adapted.

It will be appreciated that the examples of monomers that may be used togenerate cross-linked polymer networks shown in FIG. 8 are non-limitingexamples. Numerous other monomers with various functional groups, lengthof backbone, and quantity of functional groups per building block havebeen envisioned.

The monomer for forming the polymer network to provide ion selectivityin the membrane separator may be chosen based on various properties ofthe monomer. For example, the monomer may be nominated as a result ofthe conditions under which the monomer may polymerize and cure to form aresin. An electrical resistance inherent to the monomer compound,durability, selectivity towards ferric ions, ionic conductivity, andresistance to fouling may all be factors in choosing the CPG for thecross-linked polymer network.

Incorporation of the CPG into pores of the membrane separator mayinclude precipitating silica using a processing oil onto the UHMWPE. Theseparator may also include small amounts of residual oil which mayprovide a barrier to oxidation as well as carbon black, additionalantioxidants, and metal stearate as a lubricant. The membrane may beexposed to a dilute (<10 wt %) solution of the monomer. The dilution ofthe monomer solution may decrease a likelihood of augmented electricalresistivity of the membrane, resulting from precipitation of solidmaterial in the membrane pores.

Cross-linking and polymerization of the monomer may be initiated bychemical and/or thermal activation, forming the polymer network withinthe pores of the membrane. Choice of monomer used may be based onphysical conditions specific to the monomer that leads to polymerizationand curing of the resultant polymer network to produce a resin. Themembrane separator, with pores containing the resin, may be fabricatedas a continuous sheet by a calendering process. A calendering step ofthe process may allow a surface of the separator to be molded with adesired texture or pattern. For example, the surface of the separatorthat is in contact with a negative electrolyte may be imprinted withribs along the surface during the calendering step.

The ribs molded onto the surface of the separator may replace a negativespacer in the IFB. The negative spacer is included in the battery cellbetween the separator and a negative electrode to define a distancebetween the separator and negative electrode as well as to control apattern of electrolyte flow across the negative electrode and to clearhydrogen bubbles. During charging of the IFB, hydrogen (H₂) gas may begenerated within the negative electrode compartment, forming gasbubbles. The bubbles may adhere to surfaces of the negative electrode,blocking reaction sites on the electrode for iron plating and decreasingbattery efficiency. By defining flow channels along the negativeelectrode surface, the negative electrolyte may be guided to flow alongthe surface following paths which promote removal of H₂ bubbles from thesurface. The H₂ gas may be channeled to a storage tank storing negativeelectrolyte and collected in a head space of the tank, e.g., the headspace 92 of storage tank 110 of FIG. 1 .

By molding ribs in the surface of the separator, the negative spacer isintegrated into the separator, reducing manufacturing costs bydecreasing a number of components to be individually fabricated. Thecalendering process may allow a variety of rib patterns to be moldedinto the membrane separator. Examples of different rib patterns that maybe achieved by molding are shown in FIG. 9 .

A first separator surface 902, a second separator surface 904, a thirdseparator surface 906, and a fourth separator surface 908 is depicted inFIG. 9 . Each separator surface has a rib pattern that includes aplurality of ribs, each rib of the plurality of ribs separated fromadjacent ribs by a valley of a plurality of valleys. Each rib may be araised ridge, extending along the positive z-axis away from a surface ofthe separator surface and each valley may extend in an oppositedirection, along the negative z-axis. Each rib pattern presents avariation in an alignment or shape of the plurality of ribs and theplurality of valleys. Each rib of the plurality of ribs may have auniform width, measured along the x-axis and may be spaced away fromadjacent ribs by uniform widths, defined along the x-axis, of eachvalley of the plurality of valleys. In other words, the plurality ofvalleys have similar overall shape as the plurality of ribs.

A separator adapted with any of the first, second, third, and fourthseparator surfaces 902, 904, 906, and 908, may be positioned proximateto a negative electrode so that the ribbed separator surfaces are incontact with a surface of the negative electrode or placed so that thesurface of the negative electrode is spaced away from the ribbedseparator surface by a small distance. A shape of the plurality of ribsof the separator surface may influence a flow path of negativeelectrolyte along the separator surface.

In the first separator surface 902, a first set of ribs 910, interspacedwith a first set of valleys 912, may extend linearly along a length 914of the first separator surface 902, where the length 914 of the firstseparator surface 902 is also a length 914 of the second, third, andfourth separator surfaces 904, 906, 908. The length 914 may be parallelwith the y-axis and perpendicular to a width of first set of valleys 912as well as a width of the first set of ribs 910. The width of the firstset of ribs 910 may be narrower than the width of the first set ofvalleys 912. The first set of ribs 910 and first set of valleys 912 maybe arranged at an angle, such as 15 degrees, relative to the y-axis.

The second separator surface 904 may have a second set of ribs 916 and asecond set of valleys 918 with similar widths, defined along the x-axis,to the widths of the first set of ribs 910 and first set of valleys 912of the first separator surface 904. However, rather than extendinglinearly along the length 914 at an angle to the y-axis, the second setof ribs 916 and second set of valley 918 have a curving, sinuous shape.In contrast, a third set of ribs 920 and a third set of valleys 922 ofthe third separator surface 906 are aligned linearly and parallel withthe y-axis along the length 914 of the third separator surface 906.Widths of the third set of ribs 920 and third set of valleys 922 may besimilar to one another and both narrower than the widths of the firstand second sets of ribs 910, 916 and the widths of the first and secondsets of valleys 912 and 918, respectively. As a result the third set ofribs 920 may be more densely spaced than the first or second sets ofribs 910, 916, with narrower flow channels as defined by the third setof valleys 922.

The fourth separator surface 908 may have a fourth set of ribs 924 andfourth set of valleys 926 extending linearly along the length 914 of thefourth separator surface 908 at an angle to the y-axis, similar to thefirst set of ribs and valleys 910, 912 of the first separator surface902. Widths of the fourth set of ribs 924 may be wider than widths ofthe first, second and third sets of ribs 910, 916, and 920, and similarto widths of the fourth set of valleys 926. A depth of the fourth set ofribs 924, measured along the x-axis, may be defined by a distancebetween a lowest point of the each valley of the fourth set of valleys926 and a highest point of each rib of the fourth set of ribs 924, alongthe z-axis. The depth of the fourth set of ribs 924 may be greater thana depth of each of the first, second and third sets of ribs 910, 916,920 also measured along the z-axis.

By modifying a geometry of the ribs and valleys of the membraneseparator surface, flow of negative electrolyte between the separatorand negative electrode and against a surface of the negative electrodemay be moderated. The extension of the ribs and valleys along the lengthof the separator surface may guide flow along a direction parallel withthe y-axis, and may encourage release of the bubbles to float to anupper region of an electrode compartment where accumulated gas may besiphoned to an external electrolyte storage tank.

Varying an angle of the ribs molded into the separator surface, relativeto the y-axis, may allow electrolyte flow to create friction against thesurface of the negative electrode in a direction that generatesturbulent flow between the negative electrolyte and an alignment ofpores in the separator. Configuring the ribs with a sinusoidal shape,such as the second separator surface 904, may increase turbulence in theflow of negative electrolyte, increasing a likelihood that H₂ bubblesare disturbed and carried along a length of the separator. Furthermoreincreasing or decreasing a depth of the ribs may adjust a flow velocityof negative electrolyte. Thus the geometry of the ribs and valleys maybe optimized to product a desired flow pattern of negative electrolyteagainst the negative electrode. It will be appreciated that numerousvariations in geometry of the ribs and valleys of the separator may beimplemented without departing from the scope of the present disclosure.

Fabrication of a separator for an IFB with molded ribs and configuredwith a polymer network for ion selectivity may be achieved by acost-efficient method such as a calendering process. An example of acalendering system 1000 that may be used to manufacture the separator isillustrated in a schematic diagram in FIG. 10 . The calendering system1000 may be an in-line system with a series of steps and treatmentsapplied sequentially to a material. A first end 1002 of the system 1000may include a reservoir 1004 coupled to a co-rotating twin-screwextruder 1006. The reservoir 1004 may provide an inlet to the extruder1006 for materials forming the separator to enter the extruder 1006.

In one example, a mixture of UHMWPE, silica, a plasticizer such asnaphthenic oil, as well as minor amounts of carbon black, antioxidant,and calcium stearate, may be added to the reservoir 1004. An amount anda type of silica used may affect an electrical resistivity of theseparator. For example, the resistance is dependent on a combination ofresistivity of an electrolyte, a thickness of the separator, atortuosity of a pore path through the separator, and a porosity of theseparator. Increasing the porosity of the separator decreases thetortuosity of the pore path, thereby decreasing resistivity of theseparator. By increasing a proportion of silica and using silica withhigh oil absorption, an increase in separator porosity may be attained.A porosity of at least 75% may be desirable to provide sufficientlylarge pore volume to maintain low separator resistivity and sufficientlylarge pore surface areas to deposit cross-linked polymer gel(CPG)-derived polymer networks for ion selectivity.

An amount of plasticizer added to the mixture may also affect the porevolume of the separator. The plasticizer may be absorbed by the silicaand occupy spaces between submicron fibrils of the separator membranestructure. Upon extraction by solvent, described further below, removalof the plasticizer forms the voids, or pores. Increasing both the amountof silica and amount of naphthenic oil may increase overall porosity andtortuosity of the separator.

The mixture may be fed to the extruder 1006 from the reservoir 1004 andheated and compounded in the extruder 1006 so that the UHMWPE isdissolved in the oil, fully enveloping the silica. The plasticizedmixture may be ejected onto a belt 1008 and extruded through a flatsheet die 1010, in a direction indicated by arrows 1012, to form asheet. The sheet travels to a set of calender rolls 1014 that includes amolding roll 1016 configured with an outer surface that imprints adesired texture or pattern, such as the examples of ribs molded into theseparator surfaces of FIG. 9 , onto a surface of the sheet as the sheetpasses through the set of calender rolls. Ribs are molded onto thesurface of the sheet during contact with the molding roll 1016. Thesheet is also cooled and pressed to a uniform thickness as the sheet isfed through the set of calender rolls 1014.

The cooled and molded sheet is submerged in a solvent bath 1018 toremove most of the plasticizer from pores of the sheet. The sheet isremoved from the solvent bath 1018, dried and delivered to across-linked polymer gel (CPG) bath 1020 storing a dilute solution ofthe CPG dissolved in a solvent. The CPG may be derived from a variety ofmonomers, such as the examples of monomers shown in FIG. 8 , each typeof monomer adapted to interact with select ions. The sheet is removedfrom the CPG bath 1020 and treated at a curing apparatus 1024.

In one example, a chemical activator may be added to the sheet byspraying, coating or some other method of applying the chemicalactivator directly to the sheet at the curing apparatus 1024. Thechemical activator interacts with the dilute solution of the CPG storedin pores of the membrane, inducing cross-linking and polymerization toform a polymer network. In another example, polymerization may bethermally initiated and the sheet may be heated at the curing apparatus1024.

The polymer network within the pores of the membrane may be cured byheating the sheet or exposing the sheet to UV light for a period of timeat the curing apparatus 1024. Upon completion of curing, the polymernetwork forms a resin, producing the completed membrane separator. Theseparator and may be collected by winding the sheet onto a roll 1026 ata second end 1028 of the calendering process. The membrane separator maybe transported and stored as a roll. Alternatively, the sheet may beimmediately cut to desired dimensions and processed further for use in abattery system. Additional processing may include flattening,densifying, or slicing off the molded ribs along border areas of theseparator so that the separator may be sealed within a cell stack.

An example of a method 1100 for manufacturing a membrane separator foran IFB is shown in FIG. 11 . The method 1100 may be performed using acalendering system such as the calendering system 1000 of FIG. 10 . Themembrane separator may be a component in a battery cell, e.g., theseparator 24 of the battery cell 18 in FIG. 1 , of the IFB that dividesa chamber of the battery cell and separates a positive electrolyte,stored on one side of the separator, from a negative electrolyte, storedon an opposite side of the separator from the positive electrolyte. Theseparator may be configured to allow exchange of select ions across themembrane while maintaining low electrical resistivity by implementationas a relatively thin wall of material with a polymer network integratedinto pores of the separator. A negative spacer, defining a distancebetween the separator and a negative electrode immersed in the negativeelectrolyte and also guiding flow of negative electrolyte along asurface of the negative electrode, may be molded into a surface of theseparator in contact with the negative electrolyte. The negative spacermay assist in removal of gas bubbles, formed during battery charging,from the surface of the negative electrolyte and may be directlyimprinted into the surface of the separator, formed from a same materialas the separator, and produced as a unitary, continuous sheet.

At 1102, the method includes adding chemical ingredients of theseparator to an extruder of the calendering system. The extruder may bea twin-screw extruder, such as the extruder 1006 of FIG. 10 , coupled toa reservoir. The chemicals may be added to the extruder through thereservoir and may include a UHMWPE to form a base membrane for theseparator, silica and a plasticizer to create pores in the membrane, andother minor ingredients such as carbon black, an antioxidant, andcalcium stearate. Rotation of the twin-screw extruder mixes andcompounds the chemicals while heating at 1104.

The chemical mixture is extruded onto a conveyor belt and passed througha flat sheet die at 1106. The mixture emerges from the flat sheet die asa continuous sheet. At 1108, the method includes guiding the sheetthrough a set of calendering rolls. At least one roll of the set ofcalendering rolls may be adapted with an outer surface that molds apattern or texture onto a surface of the sheet, creating ribs along thesurface, such as the various examples of ribs shown in FIG. 9 . The ribsmay be configured with a geometry that results in a desired path ofnegative electrolyte flow along the negative electrode surface. Inaddition to molding ribs onto the surface of the sheet, the sheet isalso cooled and pressed to a uniform thickness by passing the sheetthrough the set of the calendering rolls.

The plasticizer may be removed at 1110 of the method. Removing theplasticizer may be achieved by submerging the molded sheet in a firstbath containing a solvent. Exposure to solvent may extract most of theplasticizer absorbed by the silica within pores of the separator, with aresidual amount of plasticizer remaining to provide a barrier tooxidation. The sheet is removed from the solvent bath and dried. Thedried sheet may be submerged in a second bath at 1112 containing adilute solution of a CPG. The CPG may comprise repeating units of amonomer, such as the first, second, and third monomers 802, 804, 806depicted in FIG. 8 , with functional groups that allow selectiveinteraction with ions. By submerging the sheet in the dilute CPGsolution, pores of the separator may be infiltrated with the CPG. Thesheet is removed from the second bath and delivered to a curing station.

At 1114, the method includes curing the CPG stored within the pores ofthe separator by exposing the CPG solution to a chemical activator or toheat. Crosslinking and polymerization is initiated in the pores, formingthe ion selective polymer network. Following polymerization, the polymernetwork may be cured at the curing station by heating the sheet.Alternatively, the polymer network may be cured by exposure to UV light.Curing the polymer network results in a transformation into a resin,completing the fabrication of the separator.

At a terminal end of the calendering system, the separator may becollected at 1118 of the method for subsequent implementation into adevice, such as the IFB. In one example, collection of the separator mayinclude winding the sheet onto a roll, cutting the sheet andtransporting the roll to be distributed and/or stored. In anotherexample, the separator sheet may be immediately cut to desireddimensions and further processed for incorporation into the IFB.

In this way, a membrane separator with an integrated negative spacer maybe manufactured for use in an iron redox flow battery (IFB) via acost-efficient method with high throughput. The separator may be formedfrom a low cost base membrane formed from an ultrahigh molecular weightpolyethylene (UHMWPE) combined with a high oil-absorbing silica and aplasticizer. Fabrication of the membrane separator may be accomplishedby a calendering system comprising a series of continuous, in-lineprocessing steps, including a calendering step, transforming rawchemicals into a continuous, unitary sheet with ribs molded into asurface of the sheet. By manufacturing the separator via the calenderingsystem, using inexpensive materials, and integrating the separator withthe negative spacer, a number of individually fabricated components forthe IFB is reduced, thereby decreasing production costs and increasingproduction output. Furthermore, a porosity of the membrane separator andgeometry of the molded ribs may be readily adjusted to achieve a desiredperformance of the IFB.

In one embodiment, a method includes combining and treating chemicalingredients of a membrane separator, forming a sheet from the combinedchemical ingredients, molding ribs onto a surface of the sheet, andinfiltrating the sheet with a cross-linked polymer network. In a firstexample of the method, combining the chemical ingredients of themembrane separator includes adding the chemical ingredients to atwin-screw extruder. A second example of the method optionally includesthe first example, and further includes, wherein treating the chemicalingredients of the membrane separator includes heating and compoundingthe chemical ingredients within the twin-screw extruder. A third exampleof the method optionally includes one or more of the first and secondexamples, and further includes, wherein forming the sheet from thecombined chemical ingredients includes extruding the chemicalingredients through a flat sheet die. A fourth example of the methodoptionally includes one or more of the first through third examples, andfurther includes, wherein forming the sheet further comprises coolingand pressing the sheet to have a uniform thickness. A fifth example ofthe method optionally includes one or more of the first through fourthexamples, and further includes, wherein molding ribs on the surface ofthe sheet includes passing the sheet through a set of calender rolls, atleast one roll of the set of calender rolls including a textured outersurface, and wherein passing the sheet through the set of calender rollsimprints texture of the textured outer surface onto the surface of thesheet. A sixth example of the method optionally includes one or more ofthe first through fifth examples, and further includes, whereininfiltrating the sheet with the cross-linked polymer network includessubmerging the sheet in a dilute solution of a cross-linked polymer gel.A seventh example of the method optionally includes one or more of thefirst through sixth examples, and further includes, wherein infiltratingthe sheet with the cross-linked polymer network further comprisesactivating polymerization of the cross-linked polymer gel and curing thepolymerized cross-linked polymer gel to form a resin within pores of theseparator, the resin comprising the cross-linked polymer network. Aneighth example of the method optionally includes one or more of thefirst through seventh examples, and further includes, fabricating abipolar plate of the redox flow battery via a roll-to-roll process, thebipolar plate also arranged in a same battery cell as the membraneseparator.

In another embodiment a separator includes a fluid-impermeable,ionically conductive membrane, and ribs molded into a surface of thefluid-impermeable, ionically conductive membrane. In a first example ofthe separator, the fluid-impermeable, ionically conductive membrane is asolid barrier between a negative electrolyte and a positive electrolyteand allows transport of selected ions between the negative electrolyteand the positive electrolyte. A second example of the separatoroptionally includes the first example, and further includes, wherein thefluid-impermeable, ionically conductive membrane blocks flow of ferrousand ferric ions between the negative electrolyte and the positiveelectrolyte. A third example of the separator optionally includes one ormore of the first and second examples, and further includes, wherein thefluid-impermeable, ionically conductive membrane is formed from amixture of a ultrahigh molecular weight polyethylene, a silica with highoil absorptivity, and a plasticizer. A fourth example of the separatoroptionally includes one or more of the first through third examples, andfurther includes, wherein a porosity of the fluid-impermeable, ionicallyconductive membrane is greater than 75%. A fifth example of theseparator optionally includes one or more of the first through fourthexamples, and further includes, wherein the fluid-impermeable, ionicallyconductive membrane includes a cross-linked polymer network within poresof the fluid-impermeable, ionically conductive membrane. A sixth exampleof the separator optionally includes one or more of the first throughfifth examples, and further includes, wherein the cross-linked polymernetwork is adapted with one or more of carbon, nitrogen and sulfur-basedfunctional groups that interact with the selected ions. A seventhexample of the separator optionally includes one or more of the firstthrough fifth examples, and further includes, wherein the ribs areformed from a same material as the fluid-impermeable, ionicallyconductive membrane and protrude outwards from the surface of thefluid-impermeable, ionically conductive membrane.

In yet another embodiment, a redox flow battery includes a first batterycell storing a positive electrolyte and a negative electrolyte within achamber of the first battery cell, a first positive electrode submergedin the positive electrolyte and a first negative electrode submerged inthe negative electrolyte of the chamber of the first battery cell, and amembrane separator arranged in the chamber of the first battery cellbetween the positive electrolyte and the negative electrolyte and incontact with the first negative electrode, the membrane separatoradapted with an ion selective polymer network disposed in pores of themembrane separator, a surface of the membrane separator forming anintegrated negative spacer in contact with the negative electrolyte. Ina first example of the battery, a bipolar plate is positioned between asecond positive electrode and a second negative electrode of a secondbattery cell adjacent to the first battery cell, the bipolar plate alsoseparating the negative electrolyte from the positive electrolyte in thesecond battery cell. A second example of the battery optionally includesthe first example, and further includes, wherein the integrated negativespacer includes a plurality of ribs and a plurality of valleys, theplurality of ribs and the plurality of valleys formed from a samematerial as the membrane separator and continuous with the membraneseparator, each rib of the plurality of ribs spaced apart by a valley ofthe plurality of valleys, the pluralities of ribs and the plurality ofvalleys defining flow paths for the negative electrolyte along a surfaceof the first negative electrode.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A separator for a redox flow battery, comprising: afluid-impermeable, ionically conductive membrane; and ribs molded into asurface of the fluid-impermeable, ionically conductive membrane.
 2. Theseparator of claim 1, wherein the fluid-impermeable, ionicallyconductive membrane is a solid barrier between a negative electrolyteand a positive electrolyte and allows transport of selected ions betweenthe negative electrolyte and the positive electrolyte.
 3. The separatorof claim 2, wherein the fluid-impermeable, ionically conductive membraneblocks flow of ferrous and ferric ions between the negative electrolyteand the positive electrolyte.
 4. The separator of claim 2, wherein theribs are configured to increase turbulent flow of the negativeelectrolyte.
 5. The separator of claim 1, wherein the fluid-impermeable,ionically conductive membrane is formed from a mixture of an ultrahighmolecular weight polyethylene, a silica with high oil absorptivity, anda plasticizer.
 6. The separator of claim 1, wherein a porosity of thefluid-impermeable, ionically conductive membrane is greater than 75%. 7.The separator of claim 1, wherein the ribs are formed from a samematerial as the fluid-impermeable, ionically conductive membrane andprotrude outwards from the surface of the fluid-impermeable, ionicallyconductive membrane.
 8. The separator of claim 1, wherein thefluid-impermeable, ionically conductive membrane includes a cross-linkedpolymer network within pores of the fluid-impermeable, ionicallyconductive membrane.
 9. The separator of claim 8, wherein thecross-linked polymer network is adapted with one or more of carbon,nitrogen and sulfur-based functional groups that interact with selectedions.
 10. The separator of claim 8, wherein the cross-linked polymernetwork is formed of monomers, the monomers including one of2-acrylamido-2-methylpropane sulfonic acid or a sodium 4-vinyl benzenesulfonate salt.
 11. The separator of claim 1, wherein the ribs extendalong a vertical axis of the separator, at an angle to the vertical axisof the separator, or sinuously across the separator.
 12. The separatorof claim 1, wherein the separator is positioned within a chamber of abattery cell of the redox flow battery.
 13. The separator of claim 12,wherein the ribs of the separator are in face-sharing contact with anegative electrode of the battery cell.
 14. A redox flow battery,comprising: a first battery cell storing a positive electrolyte and anegative electrolyte within a chamber of the first battery cell; a firstpositive electrode submerged in the positive electrolyte and a firstnegative electrode submerged in the negative electrolyte of the chamberof the first battery cell, wherein the first positive electrode isbonded to a side of a bipolar plate, the bipolar plate being aroll-to-roll calendered bipolar plate; and a membrane separator arrangedin the chamber of the first battery cell between the positiveelectrolyte and the negative electrolyte and in contact with the firstnegative electrode, the membrane separator adapted with an ion selectivepolymer network disposed in pores of the membrane separator, a surfaceof the membrane separator forming an integrated negative spacer incontact with the negative electrolyte.
 15. The redox flow battery ofclaim 14, wherein the bipolar plate is positioned between the firstpositive electrode and a second negative electrode of a second batterycell adjacent to the first battery cell, the bipolar plate separating anegative electrolyte in the second battery cell from the positiveelectrolyte in the first battery cell.
 16. The redox flow battery ofclaim 14, wherein the integrated negative spacer includes a plurality ofribs and a plurality of valleys, the plurality of ribs and the pluralityof valleys formed from a same material as the membrane separator andcontinuous with the membrane separator, each rib of the plurality ofribs spaced apart by a valley of the plurality of valleys, thepluralities of ribs and the plurality of valleys defining flow paths forthe negative electrolyte along a surface of the first negativeelectrode.
 17. The redox flow battery of claim 16, wherein the pluralityof ribs extends along a vertical axis of the integrated negative spacer,at an angle to the vertical axis of the integrated negative spacer, orsinuously across the integrated negative spacer.
 18. The redox flowbattery of claim 16, wherein the plurality of ribs are configured toincrease a turbulent flow of the negative electrolyte.
 19. The redoxflow battery of claim 14, wherein the ion selective polymer network isformed of monomers, the monomers including one of2-acrylamido-2-methylpropane sulfonic acid or a sodium 4-vinyl benzenesulfonate salt.
 20. The redox flow battery of claim 14, wherein thebipolar plate is formed of sewn together layers.